Optical glass

ABSTRACT

An optical glass having optical constants of a refractive index (nd) of 1.78 or over, an Abbe number (νd) of 30 or below, and a partial dispersion ratio (θg, F) of 0.620 or below comprises SiO 2  and Nb 2 O 5  as essential components, wherein an amount of Nb 2 O 5  in mass % is more than 40%. The optical glass further comprising, in mass % on oxide basis, less than 2% of K 2 O and one or more oxides selected from the group consisting of B 2 O 3 , TiO 2 , ZrO 2 , WO 3 , ZnO, SrO, Li 2 O and Na 2 O wherein a total amount of SiO 2 , B 2 O 3 , TiO 2 , ZrO 2 , Nb 2 O 5 , WO 3 , ZnO, SrO, Li 2 O and Na 2 O is more than 90% and TiO 2 /(ZrO 2 +Nb 2 O 5 ) is less than 0.32.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. patent application Ser. No.12/351,184, filed on Jan. 9, 2009, which claims the benefit of priorityfrom the prior Japanese Patent Application No. 2008-021643 filed on Jan.31, 2008, the entire contents of which are incorporated herein byreferences.

BACKGROUND OF THE INVENTION

This invention relates to a high refractive index, high dispersionoptical glass having a refractive index (nd) of 1.78 or over, an Abbenumber (νd) of 30 or below, and a partial dispersion ratio (θg, F) of0.620 or below. This invention relates also to optical elements such aslenses and prisms obtained by using this optical glass.

There is a large demand for a high refractive index, high dispersionoptical glass as a material for optical elements such as lenses ofvarious types. As optical glasses having a refractive index (nd) of 1.78or over and an Abbe number (νd) of 30 or below, known in the art areglass compositions disclosed by Japanese Patent Application Laid-openPublication Nos. Sho 52-25812 and 2004-161598, and WO2004/110942.

An optical system using such optical glass is mounted on an opticalproduct such as a digital camera. For improving chromatic aberration insuch optical glass, it is desired for the optical glass of a highrefractive, high dispersion region to have a relatively small partialdispersion ratio.

For this reason, from the standpoint of utility in optical design, therehas been a strong demand for an optical glass having high refractiveindex, high dispersion characteristics and a small partial dispersionratio.

Particularly, a high refractive index, high dispersion optical glasshaving a refractive index (nd) of 1.78 or over and an Abbe number (νd)of 30 or below is strongly desired for.

The above mentioned publications satisfy the above described refractiveindex and Abbe number. The optical glasses disclosed specifically inthese publications, however, do not satisfy conditions that the glasscomprises SiO₂ and Nb₂O₅ as essential components, wherein an amount ofNb₂O₅ in mass % is more than 40%, and further comprises, in mass % onoxide basis, less than 2% of K₂O and one or more oxides selected fromthe group consisting of B₂O₃, TiO₂, ZrO₂, WO₃, ZnO, SrO, Li₂O and Na₂Owherein a total amount of SiO₂, B₂O₃, TiO₂, ZrO₂, Nb₂O₅, WO₃, ZnO, SrO,Li₂O and Na₂O is more than 90% and TiO₂/(ZrO₂+Nb₂O₅ is less than 0.32.

In an optical system for a digital camera, a spherical lens is generallyused. A spherical lens is produced by heating and forming a lens preformmaterial by using a mold having a shape which is closely similar to theshape of the lens, and polishing the obtained lens preform material. Ina high refractive index, high dispersion optical glass, devitrificationtends to occur during forming of the lens preform material by heating(reheat press molding) and, therefore, an optical glass which has a highresistance to devitrification is desired for.

On the other hand, for correcting spherical surface aberration, anaspherical lens is useful. As a method for producing an aspherical lenscheaply, precision press molding is known. The precision press moldingis a method according to which glass for a lens preform material isheated and thereby softened and a high precision mold surface istransferred to the glass by pressing the mold. Since the mold is exposedto a high temperature environment, the forming surface of the mold tendsto be oxidated and eroded or a release film provided on the surface ofthe forming surface of the mold tends to be damaged and the highprecision forming surface of the mold thereby cannot be maintained orthe mold itself tends to be damaged. In such a case, the mold must beexchanged with the result that frequency of exchange of the moldincreases and a large scale production of the lens preform material at alow cost becomes difficult. For this reason, from the standpoint ofpreventing such damage to the mold, maintaining a high precision formingsurface of the mold for a long period of time and enabling precisionpress molding at a low pressing force, it is desired for an opticalglass for a lens preform material to have as low a glass transitiontemperature (Tg) as possible.

SUMMARY OF THE INVENTION

It is, therefore, an object of the invention to provide an optical glasswhich comprehensively eliminates the above described disadvantages ofthe prior art optical glasses and has the above described opticalconstants and a small partial dispersion ratio.

As a result of studies and experiments, the inventor of the presentinvention has found, which has led to the present invention, that anoptical glass having the above described optical constants and a smallpartial dispersion ratio can be obtained by adding a specific amount ofSiO₂ and Nb₂O₅ to the glass.

For achieving the above described object of the invention, in the firstaspect of the invention, there is provided an optical glass havingoptical constants of a refractive index (nd) of 1.78 or over, an Abbenumber (νd) of 30 or below, and a partial dispersion ratio (θg, F) of0.620 or below, and comprising SiO₂ and Nb₂O₅ as essential components,wherein an amount of Nb₂O₅ in mass % is more than 40%.

In the second aspect of the invention, there is provided an opticalglass as defined in the first aspect further comprising, in mass % onoxide basis, less than 2% of K₂O and one or more oxides selected fromthe group consisting of B₂O₃, TiO₂, ZrO₂, WO₃, ZnO, SrO, Li₂O and Na₂Owherein a total amount of SiO₂, B₂O₃, TiO₂, ZrO₂, Nb₂O₅, WO₃, ZnO, SrO,Li₂O and Na₂O is more than 90% and TiO₂/(ZrO₂+Nb₂O₅ is less than 0.32.

In the third aspect of the invention, there is provided an optical glassas defined in the first or second aspect comprising, in mass % on oxidebasis,

SiO₂ 10-40% Nb₂O₅ more than 40% up to 65% and B₂O₃  0-20% and/or GeO₂ 0-10% and/or Al₂O₃  0-10% and/or TiO₂  0-15% and/or ZrO₂  0-15% and/orWO₃  0-15% and/or ZnO  0-15% and/or SrO  0-15% and/or Li₂O  0-15% and/orNa₂O  0-20% and/or Sb₂O₅  0-1%.

In the fourth aspect of the invention, there is provided an opticalglass as defined in any of the first to third aspects comprising, inmass % on oxide basis,

Gd₂O₃ 0-10% and/or Y₂O₃ 0-10% and/or MgO 0-15% and/or CaO 0-15% and/orBaO 0-15% and/or Ga₂O₃ 0-10% and/or CeO₂ 0-10% and/or TeO₂ 0-10% and/orBi₂O₃ 0-10%.

In the fifth aspect of the invention, there is provided an optical glassas defined in any of the first to fourth aspects having a glasstransition point (Tg) of 650° C. or below.

In the sixth aspect of the invention, there is provided a lens preformmaterial made of an optical glass as defined in any of the first tofifth aspects.

In the seventh aspect of the invention, there is provided a lens preformmaterial for mold pressing made of an optical glass as defined in any ofthe first to sixth aspect.

In the eighth aspect of the invention, there is provided an opticalelement made of an optical glass as defined in any of the first toseventh aspects.

DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the invention will now be described.

Description will be made about respective components of the opticalglass of the present invention. Unless otherwse described, amounts ofthe respective components indicate mass % calculated on oxide basis.

In the optical glass of the invention, SiO₂ is an essential component asa glass forming oxide and it is also effective for increasing viscosityof the glass and improving resistance to devitrification and chemicaldurability. If the amount of this component is insufficient, theseeffects cannot be performed sufficiently and, if the amount of thiscomponent is excessive, resistance to devitrification and meltingproperty of the glass are deteriorated rather than improved. The lowerlimit of the amount of this component preferably is 10%, more preferably12% and most preferably 14% and the upper limit of the amount of thiscomponent is preferably 40%, more preferably 35% and most preferably30%.

SiO₂ may be introduced in the glass by using, e.g., SiO₂ as a rawmaterial.

B₂O₃ is an optional component which functions as a glass forming oxideand is also effective for reducing glass transition point (Tg). If theamount of this component is excessive, chemical durability tends to bedeteriorated. Therefore, the upper limit of the amount of this componentis preferably 20%, more preferably 15% and most preferably 10%.

B₂O₃ may be introduced into the glass by using, e.g., H₃BO₃ and B₂O₃ asraw materials.

GeO₂ is an optional component which is effective for increasingrefractive index and improving resistance to devitrification. It alsofunctions as a glass forming oxide. If the amount of this component isexcessive, the cost of the optical glass becomes high since GeO₂ is avery expensive material. Therefore, the upper limit of the amount ofthis component is preferably 10%, more preferably 5% and most preferably3%.

GeO₂ may be introduced into the glass by using, e.g., GeO₂ as a rawmaterial.

Al₂O₃ is an optional component which is effective for improving chemicaldurability. If the amount of this component is excessive, resistance todevitrification tends to be deteriorated. Therefore, the upper limit ofthe amount of this component is preferably 10%, more preferably 5% andmost preferably 3%.

Al₂O₃ may be introduced into the glass by using, e.g., Al₂O₃ andAl(OH)₃.

TiO₂ is effective for increasing refractive index and dispersion. If theamount of this component is excessive, transmittance in the visible rayshort wavelength region is deteriorated and partial dispersion ratioalso is increased. Therefore, the upper limit of the amount of thiscomponent is preferably 15%, more preferably 12% and most preferably 9%.Since TiO₂ is an optional component, the glass of the present inventioncan be produced without this component. For performing the abovedescribed effects of this component sufficiently, however, the lowerlimit of this component is preferably more than 0%, more preferably 0.1%and most preferably 1%.

TiO₂ may be introduced into the glass by using, e.g., TiO₂ as a rawmaterial.

ZrO₂ is effective for increasing refractive index and decreasing ofpartial dispersion ratio and improving chemical durability. If theamount of this component is excessive, resistance to devitrification isdeteriorated. Therefore, the upper limit of the amount of this componentis preferably 15%, more preferably 13% and most preferably 12%. SinceTiO₂ is an optional component, the glass of the present invention can beproduced without this component. For performing the above describedeffects of this component sufficiently, however, the lower limit of thiscomponent is preferably more than 0%, more preferably 0.1% and mostpreferably 1%.

ZrO₂ may be introduced into the glass by using, e.g., ZrO₂ as a rawmaterial.

Nb₂O₅ is an essential component for increasing refractive index anddispersion while reducing partial dispersion ratio and also forimproving resistance to devitrification and chemical durability. If theamount of this component is insufficient, these effects cannot beperformed sufficiently and, if the amount of this component isexcessive, resistance to devitrification of the glass is deterioratedrather than improved and transmittance in the visible ray shortwavelength region is also deteriorated. The lower limit of the amount ofthis component preferably is more than 40%, more preferably 41% and mostpreferably 42% and the upper limit of the amount of this component ispreferably 65%, more preferably 60% and most preferably 56%.

Nb₂O₅ may be introduced in the glass by using, e.g., Nb₂O₅ as a rawmaterial.

Sb₂O₃ may be optionally added for defoaming during melting of the glass.If the amount of this component is excessive, transmittance in thevisible ray short wavelength region is deteriorated. Therefore, theupper limit of the amount of this component is preferably 1%, morepreferably 0.5% and most preferably 0.2%.

Ta₂O₅ is effective for increasing refractive index and improvingchemical durability and resistance to devitrification. If the amount ofthis component is excessive, resistance to devitrification isdeteriorated rather than improved. Therefore, the upper limit of theamount of this component is preferably 15%, more preferably 12% and mostpreferably 10%.

Ta₂O₅ may be introduced into the glass by using, e.g., Ta₂O₅ as a rawmaterial.

WO₃ is effective for adjusting optical constants and improvingresistance to devitrification. If the amount of this component isexcessive, resistance to devitrification is deteriorated rather thanimproved and transmittance in the visible ray short wavelength region isdeteriorated and partial dispersion ratio is increased. Therefore, theupper limit of the amount of this component is preferably 15%, morepreferably 12% and most preferably 10%.

WO₃ may be introduced into the glass by using, e.g., WO₃ as a rawmaterial.

La₂O₃ is effective for increasing refractive index. If the amount ofthis component is excessive, resistance to devitrification isdeteriorated and it becomes difficult to provide the glass with a highdispersion characteristic. Therefore, the upper limit of the amount ofthis component is preferably 10%, more preferably 5% and most preferably3%.

La₂O₃ may be introduced into the glass by using, e.g., La₂O₃, lanthanumnitratge or its hydrate as a raw material.

Gd₂O₃ is effective for increasing refractive index. If the amount ofthis component is excessive, resistance to devitrification isdeteriorated and it becomes difficult to provide the glass with a highdispersion characteristic. Therefore, the upper limit of the amount ofthis component is preferably 10%, more preferably 5% and most preferably3%.

Gd₂O₃ may be introduced into the glass by using, e.g., Gd₂O₃ as a rawmaterial.

Yb₂O₃ is effective for increasing refractive index. If the amount ofthis component is excessive, resistance to devitrification and chemicaldurability are deteriorated and it becomes difficult to provide theglass with a high dispersion characteristic. Therefore, the upper limitof the amount of this component is preferably 10%, more preferably 5%and most preferably 3%.

Yb₂O₃ may be introduced into the glass by using, e.g., Yb₂O₃ as a rawmaterial.

Y₂O₃ is effective for increasing refractive index. If the amount of thiscomponent is excessive, resistance to devitrification is deterioratedand it becomes difficult to provide the glass with a high dispersioncharacteristic. Therefore, the upper limit of the amount of thiscomponent is preferably 10%, more preferably 5% and most preferably 3%.

Y₂O₃ may be introduced into the glass by using, e.g., Y₂O₃ as a rawmaterial.

ZnO is effective for reducing glass transition temperature (Tg) andimproving chemical durability. If the amount of this component isexcessive, resistance to devitrification is deteriorated. Therefore, theupper limit of the amount of this component is preferably 15%, morepreferably 10% and most preferably 5%.

ZnO may be introduced into the glass by using, e.g., ZnO as a rawmaterial.

MgO is effective for adjusting optical constants. If the amount of thiscomponent is excessive, resistance to devitrification is deteriorated.Therefore, the upper limit of the amount of this component is preferably15%, more preferably 10% and most preferably 5%.

MgO may be introduced into the glass by using, e.g., MgO or itscarbonate, nitrate or hydroxide as a raw material.

CaO is effective for adjusting optical constants. If the amount of thiscomponent is excessive, resistance to devitrification is deteriorated.Therefore, the upper limit of the amount of this component is preferably15%, more preferably 10% and most preferably 5%.

CaO may be introduced into the glass by using, e.g., CaO or itscarbonate, nitrate or hydroxide as a raw material.

SrO is effective for adjusting optical constants. If the amount of thiscomponent is excessive, resistance to devitrification is deteriorated.Therefore, the upper limit of the amount of this component is preferably15%, more preferably 10% and most preferably 5%.

SrO may be introduced into the glass by using, e.g., SrO or itscarbonate, nitrate or hydroxide as a raw material.

BaO is effective for adjusting optical constants. If the amount of thiscomponent is excessive, resistance to devitrification is deteriorated.Therefore, the upper limit of the amount of this component is preferably15%, more preferably 10% and most preferably 5%.

BaO may be introduced into the glass by using, e.g., BaO or itscarbonate, nitrate or hydroxide as a raw material.

Li₂O is effective for reducing partial dispersion ratio, for decreasingglass transition temperature (Tg) largely, and for enhancing melting ofglass raw materials. In the composition system of the present invention,this component is also effective for preventing devitrification duringreheat press molding. If the amount of this component is excessive,resistance to devitrification is sharply deteriorated. Therefore, theupper limit of this component is preferably 15%, more preferably 13% andmost preferably 11%. Since Li₂O is an optional component, the glass ofthe present invention can be produced without this component. Forperforming the above described effects of this component sufficiently,however, the lower limit of this component is preferably more than 0%,more preferably 0.1% and most preferably 1%.

Li₂O may be introduced into the glass by using, e.g., Li₂O or itscarbonate, nitrate or hydroxide as a raw material.

Na₂O is effective for decreasing glass transition temperature (Tg) andfor enhancing melting of mixed glass raw materials. If the amount ofthis component is excessive, resistance to devitrification is sharplydeteriorated. Therefore, the upper limit of this component is preferably20%, more preferably 15% and most preferably 13%. Since Na₂O is anoptional component, the glass of the present invention can be producedwithout this component. For performing the above described effects ofthis component sufficiently, however, the lower limit of this componentis preferably more than 0%, more preferably 0.1% and most preferably 4%.

Na₂O may be introduced into the glass by using, e.g., Na₂O or itscarbonate, nitrate or hydroxide as a raw material.

K₂O is effective for decreasing glass transition temperature (Tg) andfor enhancing melting of mixed glass raw materials. If the amount ofthis component is excessive, resistance to devitrification is sharplydeteriorated. In the composition system of the present invention,resistance to devitrification during reheat press molding is sharplydeteriorated. Therefore, the upper limit of this component is preferablyless than 2%, more preferably 1.5% and most preferably 1%.

K₂O may be introduced into the glass by using, e.g., K₂O or itscarbonate, nitrate or hydroxide as a raw material.

Bi₂O₃ is effective for increasing refractive index and decreasing glasstransition temperature (Tg). If the amount of this component isexcessive, resistance to devitrification is deteriorated and partialdispersion ratio is increased. Therefore, the upper limit of thiscomponent is preferably 10%, more preferably 5% and most preferably 3%.

Bi₂O₃ may be introduced into the glass by using, e.g., Bi₂O₃ as a rawmaterial.

TeO₂ is effective for increasing refractive index. However, in meltingglass raw materials in a platinum crucible or in a melting bath in whicha portion contacting molten glass is made of platinum, tellurium isalloyed with platinum and the alloyed portion tends to be deterioratedin heat resistance and, as a result, there is a danger that leakage ofmolten glass occurs in this portion. Therefore, the upper limit of theamount of this component is preferably 10%, more preferably 5% and mostpreferably 3%.

TeO₂ may be introduced into the glass by using, e.g., TeO₂ as a rawmaterial.

Ga₂O₃ is effective for increasing refractive index. Since, however, thiscomponent is very expensive, the upper limit of the amount of thiscomponent is preferably 10%, more preferably 5% and most preferably 3%.

Ga₂O₃ may be introduced into the glass by using, e.g., Ga₂O₃ as a rawmaterial.

CeO₂ is effective for improving resistance to devitrification. If theamount of this component is excessive, transmittance in the shortwavelength region is deteriorated. Therefore, the upper limit of theamount of this component is preferably 10%, more preferably 5% and mostpreferably 3%.

CeO₂ may be introduced into the glass by using, e.g., CeO₂ as a rawmaterial.

Raw materials which are used for introducing the respective componentsof the glass are cited for illustrative purpose and the raw materialsare not limited to those described above. Raw materials of the glasstherefore may be selected as desired from known materials according toconditions of manufacturing the glass.

The inventor of the present invention has found that a glass which has asmall partial dispersion ratio (θg, F) while having the above describedoptical constants can be obtained by adjusting the ratio of the amountof TiO₂ to the total amount of ZrO₂ and Nb₂O₃ to a defined range.Namely, the ratio of TiO₂/(ZrO₂+Nb₂O₅) should be preferably less than0.32, more preferably 0.2 and most preferably 0.15.

The inventor of the present invention has found that a high refractiveindex, high dispersion glass which has a small partial dispersion ratioand can prevent devitrification during reheat press molding can beobtained by adjusting a total amount of SiO₂, B₂O₃, TiO₂, ZrO₂, Nb₂O₅,WO₃, ZnO, SrO, Li₂O and Na₂O. Namely, the lower limit of the totalamount of SiO₂, B₂O₃, TiO₂, ZrO₂, Nb₂O₅, WO₃, ZnO, SrO, Li₂O and Na₂O ispreferably more than 90%, more preferably 91% and most preferably 94%.

For obtaining a glass which has desired optical constants and a smallpartial dispersion ratio (θg, F) and is not expensive, it is desirablethat both the ratio of TiO₂/(ZrO₂+Nb₂O₅) and the total amount of SiO₂,B₂O₃, TiO₂, ZrO₂, Nb₂O₅, WO₃, ZnO, SrO, Li₂O and Na₂O should be withinthe above described preferable ranges.

Lu₂O₃, SnO₂ and BeO may be added to the glass of the present invention.Since, however, Lu₂O₃ is an expensive material, the manufacturing costbecomes high if this component is added and, therefore, it is notpractical to use this component. SnO₂ has the disadvantage that, inmelting glass raw materials in a platinum crucible or in a melting bathin which a portion contacting molten glass is made of platinum, tin isalloyed with platinum and the alloyed portion tends to be deterioratedin heat resistance and, as a result, there is a danger that leakage ofmolten glass occurs in this portion. BeO has a harmful influence to theenvironment and therefore imposes a heavy burden to the environment.Therefore, the upper limit of the amount of each of these componentsshould be less than 0.1%, more preferably 0.05% and most preferably,these components should not be added at all.

Components which should not be added to the optical glass of the presentinvention will now be described.

A lead compound has the problem that an environmental step is necessarynot only in manufacture of the glass but also in cold processing of theglass such as polishing and disposal of the glass and therefore imposesa heavy burden to the environment.

This component therefore should not be added to the optical glass of thepresent invention.

As₂O₃, cadmium and thorium have a harmful influence to the environmentaland therefore impose a heavy burden to the environment. These componentstherefore should not be added to the optical glass of the presentinvention.

The optical glass of the present invention should preferably notcomprise coloring components such as V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Eu,Nd, Sm, Tb, Dy and Er. The term “should not comprise” herein means thatthese components should not be intentionally added and does not meanthat the glass comprises trace of these components as mixed impurities.

In the present specification, amounts of the respective componentsexpressed “on oxide basis” mean, assuming that oxides, complex salts,metal fluoride etc. used as raw materials of the glass components haveall been decomposed and converted to oxides during melting of the rawmaterials, ratios of amounts in mass % of the oxides thus produced tothe amount of the entire glass composition. In the case of a fluoride,it means an amount in mass % of the mass of F atoms actually containedto the mass of the oxide produced.

The glass composition of the present invention is expressed in mass %and cannot be converted directly to mol %. Respective componentsexisting in the glass composition satisfying the properties required inthe present invention can be expressed in mol % on oxide basis asfollows:

SiO₂ 20-50% Nb₂O₅ 10-30% or below and B₂O₃  0-20% and/or GeO₂  0-10%and/or Al₂O₃  0-10% and/or TiO₂  0-15% and/or ZrO₂  0-15% and/or WO₃ 0-10% and/or ZnO  0-10% and/or SrO  0-10% and/or Li₂O  0-40% and/orNa₂O  0-30% and/or Sb₂O₃  0-1%. Gd₂O₃  0-10% and/or Y₂O₃  0-10% and/orMgO  0-15% and/or CaO  0-15% and/or BaO  0-15% and/or Ga₂O₃  0-10%and/or CeO₂  0-10% and/or TeO₂  0-10% and/or Bi₂O₃  0-10%.

Properties of the optical glass of the present invention will now bedescribed.

As described above, from the standpoint of utility in optical design,the lower limit of refractive index (nd) of the optical glass of thepresent invention should be preferably 1.78, more preferably 1.8 andmost preferably 1.82 and the upper limit of the refractive index shouldbe preferably 1.95, more preferably 1.92 and most preferably 1.9.

From the standpoint of utility in optical design, the lower limit ofAbbe number (νd) of the optical glass of the present invention should bepreferably 18, more preferably 20 and most preferably 22 and the upperlimit of the Abbe number should be preferably 30, more preferably 28 andmost preferably 27.

From the standpoint of utility in optical design, the lower limit ofpartial dispersion ratio (6 g, F) of the optical glass of the presentinvention should be preferably 0.598, more preferably 0.600 and mostpreferably 0.602 and the upper limit of the partial dispersion ratioshould be preferably 0.620, more preferably 0.619 and most preferably0.618.

In the optical glass of the present invention, if the glass transitionpoint (Tg) is too high, deterioration in the mold etc. tends to occur inprecision press molding as described above. Therefore, Tg of the opticalglass of the present invention should be preferably 650° C., morepreferably 620° C. and most preferably 600° C.

Yield point (At) of the optical glass of the present invention should bepreferably 700° C., more preferably 670° C. and most preferably 650° C.

The optical glass of the present invention can be used as a preformmaterial for precision press molding. In the case of using the opticalglass as a preform material, the manufacturing method and method ofprecision press molding are not particularly limited but any knownmanufacturing method and precision press molding method can be used. Forexample, a preform material may be produced directly from molten glassor, alternatively, it may be produced by cold processing glass formed toa sheet.

When a preform is produced by dripping molten glass using the opticalglass of the present invention, if viscosity of molten glass is too low,striae tends to occur in the preform whereas if the viscosity is toohigh, cutting of glass becomes difficult due to self weight and surfacetension.

Accordingly, for stable production of a product of a high quality,logarithm log η of viscosity (dPa·s) at liquidus temperature shouldpreferably be within a range of 0.3-2.0, more preferably 0.4-1.8 andmost preferably 0.5-1.6.

EXAMPLES

Examples of the present invention will now be described. The inventionof course is not limited to these examples.

In Tables 1-9, compositions of Example No. 1 to No. 66 will be showntogether with refractive index (nd), Abbe number (νd), partialdispersion ratio (θg, F), glass transition temperature (Tg), yield point(At) and result of devitrification test. Amounts of the respectivecomponents are expressed in mass % on oxide basis.

In Table 10, compositions of Comparative Example No. A to No. C will beshown together with refractive index (nd), Abbe number (νd), partialdispersion ratio (θg, F), glass transition temperature (Tg), yield point(At) and result of devitrification test. Amounts of the respectivecomponents are expressed in mass % on oxide basis.

TABLE 1 Example No. 1 2 3 4 5 6 7 8 SiO₂ 26.00 21.00 26.00 26.00 24.5324.76 24.76 25.49 B₂O₃ 0.00 5.00 0.00 0.00 2.83 0.00 0.00 0.00 GeO₂ 0.000.00 0.00 0.00 0.00 0.00 0.00 0.00 Al₂O₃ 0.00 0.00 0.00 0.00 0.00 0.000.00 0.00 Gd₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Y₂O₃ 0.00 0.000.00 0.00 2.83 0.00 0.00 0.00 TiO₂ 7.00 7.00 7.00 7.00 6.60 6.67 6.676.86 ZrO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 3.92 Nb₂O₅ 51.90 51.9051.90 51.90 48.96 49.43 49.43 48.92 WO₃ 0.00 0.00 0.00 0.00 0.00 0.000.00 0.00 ZnO 0.00 0.00 0.00 0.00 0.00 4.76 0.00 0.00 MgO 0.00 0.00 0.000.00 0.00 0.00 0.00 0.00 SrO 0.00 0.00 0.00 0.00 0.00 0.00 4.76 0.00Li₂O 5.00 5.00 8.00 10.00 4.72 4.76 4.76 4.90 Na₂O 10.00 10.00 7.00 5.009.43 9.52 9.52 9.80 K₂O 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Ga₂O₃0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 CeO₂ 0.00 0.00 0.00 0.00 0.000.00 0.00 0.00 TeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Sb₂O₃ 0.100.10 0.10 0.10 0.09 0.10 0.10 0.10 Total 100 100 100 100 100 100 100 100nd 1.84971 1.85377 1.85106 1.85114 1.84021 1.85097 1.84932 1.85035 νd24.6 24.3 25.3 25.7 25.3 24.9 25.1 24.9 θg, F 0.6132 0.6142 0.61050.6086 0.6107 0.6119 0.6114 0.6113 Tg (° C.) At (° C.) TiO₂/(ZrO₂ +Nb₂O₅) 0.13 0.13 0.13 0.13 0.13 0.13 0.13 0.13 SiO₂, B₂O₃, TiO₂, 99.9099.90 99.90 99.90 97.08 99.90 99.90 99.90 ZrO₂, Nb₂O₅, WO₃, ZnO, SrO,Li₂O, Na₂O Total amount (%) Devitrification test (600° C.)

TABLE 2 Example No. 9 10 11 12 13 14 15 16 SiO₂ 25.00 24.53 25.24 25.2424.53 23.64 24.76 25.24 B₂O₃ 0.00 2.83 0.00 0.00 0.00 0.00 0.00 0.00GeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2.91 Al₂O₃ 0.00 0.00 2.91 0.000.00 0.00 0.00 0.00 Gd₂O₃ 0.00 2.83 0.00 0.00 0.00 0.00 0.00 0.00 Y₂O₃0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 TiO₂ 6.73 6.60 6.80 6.80 6.606.36 6.67 6.80 ZrO₂ 7.69 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Nb₂O₅ 46.0648.96 50.39 50.39 48.96 47.18 49.43 50.39 WO₃ 0.00 0.00 0.00 2.91 5.669.09 0.00 0.00 ZnO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 MgO 0.00 0.000.00 0.00 0.00 0.00 4.76 0.00 SrO 0.00 0.00 0.00 0.00 0.00 0.00 0.000.00 Li₂O 4.81 4.72 4.85 4.85 4.72 4.55 4.76 4.85 Na₂O 9.62 9.43 9.719.71 9.43 9.09 9.52 9.71 K₂O 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Ga₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 CeO₂ 0.00 0.00 0.00 0.000.00 0.00 0.00 0.00 TeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Sb₂O₃0.10 0.09 0.10 0.10 0.09 0.09 0.10 0.10 Total 100 100 100 100 100 100100 100 nd 1.85095 1.83968 1.83150 1.85027 1.85213 1.85483 1.835281.84425 νd 25.3 25.2 25.1 24.5 24.4 24.3 25.9 24.8 θg, F 0.6093 0.61260.6116 0.6135 0.6142 0.6150 0.6078 0.6126 Tg (° C.) At (° C.)TiO₂/(ZrO₂ + Nb₂O₅) 0.13 0.13 0.13 0.13 0.13 0.13 0.13 0.13 SiO₂, B₂O₃,TiO₂, 99.90 97.08 96.99 99.90 99.91 99.91 95.14 96.99 ZrO₂, Nb₂O₅, WO₃,ZnO, SrO, Li₂O, Na₂O Total amount (%) Devitrification test (600° C.)

TABLE 3 Example No. 17 18 19 20 21 22 23 24 SiO₂ 25.24 25.24 25.24 25.2425.24 28.16 25.24 24.76 B₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 2.91 0.00GeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Al₂O₃ 0.00 0.00 0.00 0.000.00 0.00 0.00 0.00 Gd₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Y₂O₃0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 TiO₂ 6.80 6.80 6.80 6.80 6.806.80 6.80 6.67 ZrO₂ 0.00 0.00 0.00 0.00 2.91 0.00 0.00 10.48 Nb₂O₅ 50.3950.39 50.39 53.30 50.39 50.39 50.39 43.71 WO₃ 0.00 0.00 0.00 0.00 0.000.00 0.00 0.00 ZnO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 MgO 0.00 0.000.00 0.00 0.00 0.00 0.00 0.00 SrO 0.00 0.00 0.00 0.00 0.00 0.00 0.000.00 Li₂O 4.85 4.85 4.85 4.85 4.85 4.85 4.85 4.76 Na₂O 9.71 9.71 9.719.71 9.71 9.71 9.71 9.52 K₂O 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Ga₂O₃ 2.91 0.00 0.00 0.00 0.00 0.00 0.00 0.00 CeO₂ 0.00 2.91 0.00 0.000.00 0.00 0.00 0.00 TeO₂ 0.00 0.00 2.91 0.00 0.00 0.00 0.00 0.00 Sb₂O₃0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 Total 100 100 100 100 100 100100 100 nd 1.84380 1.84839 1.84626 1.85785 1.85345 1.83562 1.839261.84961 νd 24.8 24.7 24.6 24.2 24.7 25.1 24.9 25.7 θg, F 0.6124 0.61270.6137 0.6146 0.6120 0.6114 0.6123 0.6080 Tg (° C.) At (° C.)TiO₂/(ZrO₂ + Nb₂O₅) 0.13 0.13 0.13 0.13 0.13 0.13 0.13 0.12 SiO₂, B₂O₃,TiO₂, 96.99 96.99 96.99 99.90 99.90 99.90 99.90 99.90 ZrO₂, Nb₂O₅, WO₃,ZnO, SrO, Li₂O, Na₂O Total amount (%) Devitrification test (600° C.)

TABLE 4 Example No. 25 26 27 28 29 30 31 32 SiO₂ 24.76 24.53 25.00 25.2424.76 15.24 25.00 25.24 B₂O₃ 0.00 0.00 0.00 0.00 0.00 9.52 0.00 0.00GeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Al₂O₃ 0.00 0.00 0.00 0.000.00 0.00 0.00 0.00 Gd₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Y₂O₃0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 TiO₂ 6.67 6.60 5.77 4.85 5.715.71 6.73 6.80 ZrO₂ 8.57 9.43 8.65 8.74 8.57 8.57 9.62 9.71 Nb₂O₅ 45.6245.19 46.06 46.50 46.57 46.57 46.06 46.50 WO₃ 0.00 0.00 0.00 0.00 0.000.00 0.00 0.00 ZnO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 MgO 0.00 0.000.00 0.00 0.00 0.00 0.00 0.00 SrO 0.00 0.00 0.00 0.00 0.00 0.00 0.000.00 Li₂O 4.76 4.72 4.81 4.85 4.76 4.76 2.88 4.85 Na₂O 9.52 9.43 9.629.71 9.52 9.52 9.62 6.80 K₂O 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Ga₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 CeO₂ 0.00 0.00 0.00 0.000.00 0.00 0.00 0.00 TeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Sb₂O₃0.10 0.09 0.10 0.10 0.10 0.10 0.10 0.10 Total 100 100 100 100 100 100100 100 nd 1.85253 1.85416 1.84754 1.84266 1.85077 1.85381 1.862441.87031 νd 25.3 25.4 25.6 25.9 25.5 25.2 24.5 24.7 θg, F 0.6089 0.60790.6056 0.6080 0.6095 0.6124 0.6117 Tg (° C.) 561 587 574 At (° C.) 605630 619 TiO₂/(ZrO₂ + Nb₂O₅) 0.12 0.12 0.11 0.09 0.10 0.10 0.12 0.12SiO₂, B₂O₃, TiO₂, 99.90 99.91 99.90 99.90 99.90 99.90 99.90 99.90 ZrO₂,Nb₂O₅, WO₃, ZnO, SrO, Li₂O, Na₂O Total amount (%) Devitrification test(600° C.)

TABLE 5 Example No. 33 34 35 36 37 38 39 40 SiO₂ 14.95 15.53 24.88 15.2415.53 15.53 15.09 14.41 B₂O₃ 9.35 9.71 0.00 9.52 9.71 9.71 9.43 9.01GeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Al₂O₃ 0.00 0.00 0.00 0.000.00 0.00 0.00 0.00 Gd₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Y₂O₃0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 TiO₂ 5.61 4.85 4.78 3.81 1.941.94 0.00 0.00 ZrO₂ 10.28 8.74 8.61 10.48 10.68 10.68 10.38 9.91 Nb₂O₅45.70 46.50 47.27 46.57 47.48 47.48 50.85 53.06 WO₃ 0.00 0.00 0.00 0.000.00 0.00 0.00 0.00 ZnO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 MgO 0.000.00 0.00 0.00 0.00 0.00 0.00 0.00 SrO 0.00 0.00 0.00 0.00 0.00 0.000.00 0.00 Li₂O 4.67 4.85 4.78 4.76 4.85 1.94 4.72 4.50 Na₂O 9.35 9.719.57 9.52 9.71 12.62 9.43 9.01 K₂O 0.00 0.00 0.00 0.00 0.00 0.00 0.000.00 Ga₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 CeO₂ 0.00 0.00 0.000.00 0.00 0.00 0.00 0.00 TeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Sb₂O₃ 0.09 0.10 0.10 0.10 0.10 0.10 0.09 0.09 Total 100 100 100 100 100100 100 100 nd 1.85682 1.84556 1.84776 1.84684 1.83688 1.83318 1.843011.85818 νd 25.3 25.7 25.7 25.9 26.6 25.8 26.4 25.7 θg, F 0.6087 0.60730.6072 0.6065 0.6034 0.6058 0.6041 0.6067 Tg (° C.) 501 532 At (° C.)547 573 TiO₂/(ZrO₂ + Nb₂O₅) 0.10 0.09 0.09 0.07 0.03 0.03 0.00 0.00SiO₂, B₂O₃, TiO₂, 99.91 99.90 99.90 99.90 99.90 99.90 99.91 99.91 ZrO₂,Nb₂O₅, WO₃, ZnO, SrO, Li₂O, Na₂O Total amount (%) Devitrification test(600° C.)

TABLE 6 Example No. 41 42 43 44 45 46 47 48 SiO₂ 14.95 24.53 24.07 15.2415.24 15.09 15.24 14.61 B₂O₃ 9.35 0.00 0.00 9.52 9.52 9.43 9.52 9.13GeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Al₂O₃ 0.00 0.00 0.00 0.000.00 0.00 0.00 0.00 Gd₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Y₂O₃0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 TiO₂ 5.61 6.60 6.48 5.71 5.716.60 5.71 7.76 ZrO₂ 10.28 0.00 0.00 8.57 8.57 0.00 0.00 10.05 Nb₂O₅45.70 54.62 55.46 46.57 46.57 54.62 55.14 44.66 WO₃ 0.00 0.00 0.00 0.000.00 0.00 0.00 0.00 ZnO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 MgO 0.000.00 0.00 0.00 0.00 0.00 0.00 0.00 SrO 0.00 0.00 0.00 0.00 0.00 0.000.00 0.00 Li₂O 1.87 4.72 4.63 4.76 4.76 4.72 4.76 1.83 Na₂O 12.15 9.439.26 9.52 9.52 9.43 9.52 11.87 K₂O 0.00 0.00 0.00 0.00 0.00 0.00 0.000.00 Ga₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 CeO₂ 0.00 0.00 0.000.00 0.00 0.00 0.00 0.00 TeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Sb₂O₃ 0.09 0.09 0.09 0.10 0.10 0.09 0.10 0.09 Total 100 100 100 100 100100 100 100 nd 1.85245 1.86874 1.87448 1.84286 1.85303 1.87293 1.868291.86405 νd 24.6 23.8 23.6 25.6 25.3 23.5 23.7 24.0 θg, F 0.6112 0.61560.6166 0.6084 0.6093 0.6175 0.6159 0.6147 Tg (° C.) At (° C.)TiO₂/(ZrO₂ + Nb₂O₅) 0.10 0.12 0.12 0.10 0.10 0.12 0.10 0.14 SiO₂, B₂O₃,TiO₂, 99.91 99.91 99.91 99.90 99.90 99.91 99.90 99.91 ZrO₂, Nb₂O₅, WO₃,ZnO, SrO, Li₂O, Na₂O Total amount (%) Devitrification test (600° C.)

TABLE 7 Example No. 49 50 51 52 53 54 55 56 SiO₂ 14.16 24.07 23.85 24.0723.85 14.47 14.31 23.42 B₂O₃ 8.85 0.00 0.00 0.00 0.00 9.04 8.94 0.00GeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Al₂O₃ 0.00 0.00 0.00 0.000.00 0.00 0.00 0.00 Gd₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Y₂O₃0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 TiO₂ 5.31 6.48 6.42 6.48 6.426.87 6.08 6.31 ZrO₂ 9.73 1.85 2.75 1.85 2.75 9.95 9.84 4.50 Nb₂O₅ 48.5853.61 53.12 53.61 53.12 46.02 47.32 52.16 WO₃ 0.00 0.00 0.00 0.00 0.000.00 0.00 0.00 ZnO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 MgO 0.00 0.000.00 0.00 0.00 0.00 0.00 0.00 SrO 0.00 0.00 0.00 0.00 0.00 0.00 0.000.00 Li₂O 1.77 4.63 4.59 1.85 1.83 1.81 1.79 1.80 Na₂O 11.50 9.26 9.1712.04 11.93 11.75 11.63 11.71 K₂O 0.00 0.00 0.00 0.00 0.00 0.00 0.000.00 Ga₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 CeO₂ 0.00 0.00 0.000.00 0.00 0.00 0.00 0.00 TeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Sb₂O₃ 0.09 0.09 0.09 0.09 0.09 0.09 0.09 0.09 Total 100 100 100 100 100100 100 100 nd 1.87040 1.87183 1.87332 1.86873 1.87012 1.86589 1.868151.87275 νd 23.9 23.9 23.9 23.2 23.3 24.0 24.0 23.3 θg, F 0.6143 0.61520.6146 0.6187 0.6176 0.6141 0.6143 0.6170 Tg (° C.) At (° C.)TiO₂/(ZrO₂ + Nb₂O₅) 0.09 0.12 0.11 0.12 0.11 0.12 0.11 0.11 SiO₂, B₂O₃,TiO₂, 99.91 99.91 99.91 99.91 99.91 99.91 99.91 99.91 ZrO₂, Nb₂O₅, WO₃,ZnO, SrO, Li₂O, Na₂O Total amount (%) Devitrification test (600° C.)

TABLE 8 Example No. 57 58 59 60 61 62 63 64 SiO₂ 23.21 15.38 15.31 14.9514.81 22.81 22.41 23.21 B₂O₃ 0.00 9.62 9.57 9.35 9.26 0.00 0.00 0.00GeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Al₂O₃ 0.00 0.00 0.00 0.000.00 0.00 0.00 0.00 Gd₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Y₂O₃0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 TiO₂ 6.25 4.81 5.26 5.61 5.566.14 6.03 6.25 ZrO₂ 5.36 0.00 0.00 1.87 2.78 7.02 8.62 5.36 Nb₂O₅ 51.7055.67 55.41 54.11 53.61 50.79 49.91 51.70 WO₃ 0.00 0.00 0.00 0.00 0.000.00 0.00 0.00 ZnO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 MgO 0.00 0.000.00 0.00 0.00 0.00 0.00 0.00 SrO 0.00 0.00 0.00 0.00 0.00 0.00 0.000.00 Li₂O 1.79 4.81 4.78 4.67 4.63 1.75 1.72 4.46 Na₂O 11.61 9.62 9.579.35 9.26 11.40 11.21 8.93 K₂O 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Ga₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 CeO₂ 0.00 0.00 0.00 0.000.00 0.00 0.00 0.00 TeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Sb₂O₃0.09 0.10 0.10 0.09 0.09 0.09 0.09 0.09 Total 100 100 100 100 100 100100 100 nd 1.87421 1.86304 1.86543 1.87067 1.87202 1.87685 1.879271.87772 νd 23.4 24.0 23.9 23.8 23.9 23.5 23.6 24.0 θg, F 0.6172 0.61580.6164 0.6158 0.6150 0.6165 0.6156 0.6141 Tg (° C.) 559 At (° C.) 605TiO₂/(ZrO₂ + Nb₂O₅) 0.11 0.09 0.09 0.10 0.10 0.11 0.10 0.11 SiO₂, B₂O₃,TiO₂, 99.91 99.90 99.90 99.91 99.91 99.91 99.91 99.91 ZrO₂, Nb₂O₅, WO₃,ZnO, SrO, Li₂O, Na₂O Total amount (%) Devitrification ∘ test (600° C.)

TABLE 9 Example No. 65 66 SiO₂ 22.81 22.61 B₂O₃ 0.00 0.00 GeO₂ 0.00 0.00Al₂O₃ 0.00 0.00 Gd₂O₃ 0.00 0.00 Y₂O₃ 0.00 0.00 TiO₂ 7.89 6.09 ZrO₂ 5.265.22 Nb₂O₅ 50.79 52.96 WO₃ 0.00 0.00 ZnO 0.00 0.00 MgO 0.00 0.00 SrO0.00 0.00 Li₂O 1.75 1.74 Na₂O 11.40 11.30 K₂O 0.00 0.00 Ga₂O₃ 0.00 0.00CeO₂ 0.00 0.00 TeO₂ 0.00 0.00 Sb₂O₃ 0.09 0.09 Total 100 100 nd 1.883291.88267 νd 22.9 23.1 θg, F 0.6187 0.6184 Tg At TiO₂/(ZrO₂ + Nb₂O₅) 0.140.10 SiO₂, B₂O₃, TiO₂, ZrO₂, 99.91 99.91 Nb₂O₅, WO₃, ZnO, SrO, Li₂O,Na₂O Total amount(%) Devitrification test (600° C.)

TABLE 10 Example No. A B C SiO₂ 21.00 28.00 22.00 B₂O₃ 2.00 0.00 0.00GeO₂ 0.00 0.00 0.00 Al₂O₃ 0.00 1.00 0.00 La₂O₃ 3.00 0.00 0.00 Y₂O₃ 0.000.00 0.00 TiO₂ 12.00 13.00 32.00 ZrO₂ 6.00 0.00 1.00 Nb₂O₅ 38.90 44.0013.00 WO₃ 2.00 0.00 0.00 ZnO 0.00 0.00 0.00 CaO 2.00 0.00 0.00 SrO 0.000.00 0.00 BaO 4.00 2.00 17.00 Li₂O 6.00 6.00 0.00 Na₂O 0.00 0.00 14.95K₂O 3.00 6.00 0.00 Ga₂O₃ 0.00 0.00 0.00 CeO₂ 0.00 0.00 0.00 TeO₂ 0.000.00 0.00 Sb₂O₃ 0.10 0.00 0.05 Total 100.00 100.00 100.00 nd 1.897331.84093 1.84725 νd 24.4 24.1 23.8 θg, F 0.6143 0.6180 0.6201 Tg 552 550588 At 601 593 628 TiO₂/(ZrO₂ + Nb₂O₅) 0.27 0.30 2.29 SiO₂, B₂O₃, TiO₂,ZrO₂, Nb₂O₅, 87.90 91.00 82.95 WO₃, ZnO, SrO, Li₂O, Na₂O Total amount(%) Devitrification ◯ test 690° C. Devitrification X X ◯ test 660° C.

For producing Examples No. 1 to No. 66 of the optical glass of thepresent invention shown in Tables 1-9, optical glass raw materials suchas oxides, hydroxides, carbonates and nitrates were weighed at thecomposition ratios of the respective Examples and mixed together andthen were put in a platinum crucible. The raw materials were melted at1100° C. to 1400° C. for 3 hours to 5 hours according to meltingproperty of the materials of the respective compositions and thenrefined and stirred for homogenization. Then, the molten glass was castin a mold and annealed to products.

Refractive index (nd) and Abbe number (νd) were measured with respect tothe optical glasses which were obtained by using annealing temperaturefalling speed of −25° C./hour.

For calculating partial dispersion ratio (6 g, F), refractive index (nC)at C line (wavelength of 656.27 nm), refractive index (nF) at F line(wavelength of 486.13 nm) and refractive index (ng) at g line(wavelength of 435.835 nm) were measured with respect to the opticalglasses obtained by using annealing temperature falling speed of −25°C./hour and the partial dispersion ratio was calculated by using theformula θg, F=(ng−nF)/(nF−nC).

Glass transition temperature (Tg) was measured by using the methoddescribed in Japan Optical Glass Industry Standard JOGIS08-2003(Measuring method of thermal expansion of optical glass). As a testpiece, a specimen having length of 50 mm and diameter of 4 mm was used.

Yield point (At) was measured by using a measuring method similar to themeasuring method of glass transition temperature (Tg) and a temperatureat which the glass ceased to stretch and started to shrink wasdetermined as yield point.

For conducting the devitrification test, a piece of glass obtained bycutting glass gob to a size of 10-40 mm was used as a test piece. Thetest piece was put in an electric furnace and temperature of theelectric furnace was raised to a predetermined temperature for 1 hour to3 hours and the test piece was held at the predetermined temperature for30 minutes and cooled in the electric furnace. Then, the glass piece waspolished on both surfaces and devitrification in the test piece wasobserved with eye and by a microscope. As a result of the observation, atest piece in which devitrification was not observed was marked with ◯and a test piece in which devitrification was observed was marked withX.

As will be noted from Tables 1-9, the Example No. 1 to No. 66 of theoptical glass of the present invention all have optical constants(refractive index and Abbe number) within the above described ranges,have partial dispersion ratio (θg, F) of 0.620 or below, and have nodevitrification in the glass by the devitrification test at 660° C.Further, since glass transition temperature (Tg) of these Examples is650° C. or below, they are suitable for precision press molding.

In contrast, glasses were produced with respect to the glasscompositions of Comparative Example No. A to No. C shown in Table 10 byemploying the same conditions as in the Examples and the producedglasses were assessed by the same method as in the Examples. As will benoted from Table 10, the Comprative Examples No. A and B havedevitrification by the devitrification test because they have K₂O in anamount which is out of the range of less than 2%. In Comparative ExampleC, since TiO₂/(ZrO₂+Nb₂O₅) is out of the range of less than 0.32, andthe total amount of SiO₂, B₂O₃, TiO₂, ZrO₂, Nb₂O₅, WO₃, ZnO, SrO, Li₂Oand Na₂O is out of the range of more than 90%, partial dispersion ratio(θg, F) does not satisfy the value of 0.620 or below. Therefore, theglasses of the Comparative Examples cannot be used for industrialpurposes.

INDUSTRIAL UTILITY

As described in the foregoing, since the optical glass of the presentinvention is of a SiO₂—Nb₂O₅ system, comprises Nb₂O₅ in an amount ofmore than 40% in mass %, has refractive index (nd) of 1.78 or over andAbbe number (νd) of 30 or below, and has partial dispersion ratio of0.620 or below, it is very useful in optical design. Further, since theoptical glass has glass transition temperature (Tg) of 650° C. or below,it is suitable for precision press molding. Thus, the optical glass ofthe present invention is very useful for industrial purposes.

1. An optical glass having optical constants of a refractive index (nd)of 1.78 or over, an Abbe number (νd) of 30 or below, and a partialdispersion ratio (θg, F) of 0.620 or below, and comprising SiO₂ andNb₂O₅ as essential components, wherein an amount of Nb₂O₅ in mass % ismore than 40%.
 2. The optical glass as defined in claim 1 furthercomprising, in mass % on oxide basis, less than 2% of K₂O and one ormore oxides selected from the group consisting of B₂O₃, TiO₂, ZrO₂, WO₃,ZnO, SrO, Li₂O and Na₂O wherein a total amount of SiO₂, B₂O₃, TiO₂,ZrO₂, Nb₂O₅, WO₃, ZnO, SrO, Li₂O and Na₂O is more than 90% andTiO₂/(ZrO₂+Nb₂O₅) is less than 0.32.
 3. The optical glass as defined inclaim 1 comprising, in mass % on oxide basis: SiO₂ 10-40%, Nb₂O₅ morethan 40% up to 65%, B₂O₃  0-20%, GeO₂  0-10%, Al₂O₃  0-10%, TiO₂  0-15%,ZrO₂  0-15%, WO₃  0-15%, ZnO  0-15%, SrO  0-15%, Li₂O  0-15%, Na₂O 0-20%, and Sb₂O₅  0-1%.


4. The optical glass as defined in claim 1, further comprising one ormore materials, in mass % on oxide basis: Gd₂O₃ 0-10%, Y₂O₃ 0-10%, MgO0-15%, CaO 0-15%, BaO 0-15%, Ga₂O₃ 0-10%, CeO₂ 0-10%, TeO₂ 0-10%, andBi₂O₃ 0-10%.


5. The optical glass as defined in claim 3, further comprising one ormore materials, in mass % on oxide basis: Gd₂O₃ 0-10%, Y₂O₃ 0-10%, MgO0-15%, CaO 0-15%, BaO 0-15%, Ga₂O₃ 0-10%, CeO₂ 0-10%, TeO₂ 0-10%, andBi₂O₃ 0-10%.


6. The optical glass as defined in claim 1 having a glass transitionpoint (Tg) of 650° C. or below.
 7. The optical glass as defined in claim1, wherein the optical glass is a lens preform.
 8. The optical glass asdefined in claim 1, wherein the optical glass is a lens preform forpress molding.
 9. The optical glass as defined in claim 1, wherein theoptical glass is included in an optical element.